This invention relates to balun transformers which are utilized for matching an unbalanced line to a balanced line. More particularly, the invention relates to a microstrip balun transformers and antennas.
In conventional radio communications applications, for example in portable and paging radio systems, antennas are used which are often fabricated by processes which are only partially automated. Unfortunately, such antennas must often still be manually adjusted or "trimmed" to the desired operating frequency. Such manual trimming and the attendant manual testing of the antenna is labor intensive and thus adds significant expense to the manufactured antenna product.
For example, prior antennas such as the half wave sleeve dipole antenna 10 of FIG. 1 are mechanically relatively complex and require manual antenna adjustment and testing to bring the antenna to the desired antenna operating frequency. The detailed structure of a typical sleeve dipole antenna is set forth below such that the complexity of manufacturing and tuning such an antenna may be fully appreciated.
In such an antenna, a wire radiator 20, which exhibits a length equivalent to approximately one-fourth wavelength in air, is fed by the inner conductor 25 of a coaxial transmission line 30. A dielectric insulator 32 separates inner conductor 25 from outer conductor 35. The outer conductor 35 of coaxial transmission line 30 is electrically coupled to feed a metallic sleeve 40 which is also approximately one quarter wavelength long in air. To improve the compactness of this antenna structure, metallic sleeve 40 is normally disposed about a portion of coaxial transmission line 30, with a uniform dielectric spacer 45 positioned to maintain the proper physical relationship between the coaxial line 30 and the metallic sleeve 40. Dielectric spacer 45 is generally cylindrical in shape and serves to establish an outer transmission line 47 wherein the outer conductor is metallic sleeve 40 an the inner conductor is the outer conductor 35 of coaxial transmission line 30. This outer transmission line is approximately one quarter of a wavelength in the dielectric material of spacer 45. A connector 55 is coupled to coaxial line 30 to facilitate connection of the antenna to radio devices.
Element 20 is typically cut during manufacture to a length which brings the resultant manufactured antenna to a frequency slightly lower than the desired operating frequency of the antenna. Additional manual frequency testing and trimming is then required to tune the antenna of FIG. 1 to the desired operating frequency. As already discussed, such additional steps are very expensive due to their manual nature. It is clear that antennas which avoid these steps are very desirable. It is also clear that antennas which are mechanically less complex are very desirable.